The present invention is especially useful in electronic ballast circuits for lighting applications, but is also useful in other electronic circuits where space and cost savings are desired. Although the invention will be described in connection with electronic ballast circuits for lighting applications, it is applicable to other electrical and electronic circuits as well.
In an electronic ballast which has an energy storage device, capacitors are often used to hold a charge. In many electronic ballasts, a dc bus voltage is created in order to efficiently drive the lamps. The bus voltage must be greater than the peak voltage of the input line in order for the power factor correction boost converter to work properly. For any input line, the peak voltage is the product of the nominal input line voltage and the square root of two (which is approximately 1.414). For a standard 120 Vac input line, which is a common household voltage, the peak voltage of the line is approximately 170 V. A typical dc bus voltage is 230 Vdc. The capacitors that support this voltage must therefore be rated for at least 230 Vdc. Capacitors are normally chosen so that the voltage across them is between 65% and 95% of their rated voltage. This range of values is recommended by the capacitor manufacturer. If the capacitor continually sees voltages above this range (i.e., voltages above 95% of rated voltage), the life of the capacitor is reduced. If the capacitor continually sees voltages below this range (i. e., voltages below 65% of rated voltage), the dielectric of the capacitor reforms and the capacitor's maximum voltage rating is decreased to the lower voltage value.
In the previous example, typically two 160 Vdc-rated or two 200 Vdc-rated capacitors, giving a total rating of 320 Vdc or 400 Vdc, would be connected in series to provide a safety margin above 230 Vdc.
For 200 Vac, 220 Vac, and 240 Vac lines, which are common voltages in Asia and Europe, the peak of the line is about 340 V. A typical bus voltage is around 410 Vdc. The capacitors that support this voltage are generally rated for 450 Vdc, so that either a single 450 Vdc or two 250 Vdc capacitors in series are used.
For a 277 Vac line, which is a common voltage in industrial and commercial buildings in the United States, the peak of the line is approximately 400 V, and a typical bus voltage is around 450 Vdc. In order to achieve some safety margin, a capacitor with a total voltage rating of at least 500 Vdc is required. Capacitors with a voltage rating in excess of 450 Vdc are expensive, take up a lot of space, and are difficult to obtain.
In the field of electronic ballasts, and indeed in many other fields, physical size and total cost are important, if not the main, design considerations. In order to provide a capacitor for a 500 Vdc bus, design engineers typically would use two 250 Vdc capacitors in series. Capacitors with voltage ratings of 250 Vdc or greater often must be inserted into printed circuit boards by hand, on their side, and then secured to the printed circuit board with either tape, adhesive, or ties. The capacitor leads extend outwardly from the end of the capacitors, and must be bent at a right angle in order to be connected to the printed circuit board. This adds to increased manufacturing and assembly costs, decreased reliability, and adds to the area the capacitors take up on the printed circuit board.